Orthogonal frequency division multiplexing system with superframe synchronization using correlation sequence

ABSTRACT

A wireless transmitter (TX 1 ). The transmitter comprises circuitry for providing a plurality of control bits (CONTROL) and circuitry for providing a plurality of user bits (USER). The transmitter further comprises circuitry ( 16 ) for modulating the plurality of control bits and the plurality of user bits into a stream of complex symbols and circuitry ( 18 ) for converting the stream of complex symbols into a parallel plurality of complex symbol streams. The transmitter further comprises circuitry ( 20 ) for performing an inverse fast Fourier transform on the parallel plurality of complex symbol streams to form a parallel plurality of OFDM symbols and circuitry ( 22 ) for converting the parallel plurality of OFDM symbols into a serial stream of OFDM symbols. The serial stream consists of an integer N+1 OFDM symbols. Each OFDM symbol in the serial stream of OFDM symbols comprises a plurality of data points. Finally, selected OFDM symbols (SF 1.x ′) in the serial stream of OFDM symbols each comprise control data points carrying a portion of a synchronization code.

CROSS-REFERENCES TO RELATED APPLICATIONS

Not Applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND OF THE INVENTION

The present embodiments relate to wireless communications systems andare more particularly directed to an orthogonal frequency divisionmultiplexing (“OFDM”) system.

Wireless communications are now prevalent in many applications,including both business and personal communication systems. The presentembodiments have particular application in such systems and particularlythose that are sometimes referred to as fixed wireless systems. Fixedwireless systems are so named because the last distance of the downlinkcommunication, typically on the order of one or two miles, is expectedto include a wireless communication to a device that is not mobile or,if mobile, has a very slow fading characteristic. For example, a fixedwireless system in contemporary applications may include wirelesscommunications to a modem inside a home or business.

One wireless technique that has had favorable use in a fixed environmentis OFDM, which also is introduced here as it has particular applicationto the preferred embodiments described later. By way of introduction toOFDM, the more general frequency division multiplexing (“FDM”) ischaracterized by transmission of multiple signals simultaneously over asingle transmission path, such as a wireless system. Each of themultiple signals travels at a different frequency band, sometimesreferred to as a carrier or sub-carrier, and which is modulated by thedata. More particularly, each sub-carrier is actually a sinc (sin(x)/x)function. In any event, the data carried by each sub-carrier may be userdata of many forms, including text, voice, video, and the like. Inaddition, the data includes control data, a particular type of which isdiscussed below. In any event, OFDM was developed several years ago, andit adds an element of orthogonality to FDM. In OFDM, the centerfrequency of each of the sub-carriers are spaced apart at specificfrequencies, where the frequency spacing is such that each sub-carrieris orthogonal to the other sub-carriers. As a result of theorthogonality, ideally each receiving element tuned to a givensub-carrier does not perceive any of the signals communicated at anyother of the sub-carriers. Given this aspect, various benefits arise.For example, OFDM is able to use overlapping (while orthogonal)sub-carriers and, as a result, thorough use is made of the overall OFDMspectrum. As another example, in many wireless systems, the sametransmitted signal arrives at the receiver at different times, that is,having traveled different lengths due to reflections in the channelbetween the transmitter and receiver; each different arrival of the sameoriginally-transmitted signal is typically referred to as a multipath.Typically multipaths interfere with one another, which is sometimesreferred to as intersymbol interference (“ISI”) because each pathincludes transmitted data referred to as symbols. Nonetheless, theorthogonality implemented by OFDM considerably reduces ISI and, as aresult, often a less complex receiver structure, such as one without anequalizer, may be implemented in an OFDM system. Lastly, note that OFDMalso has been used in mobile wireless communications, and is currentlybeing developed in various respects including in combination with otherwireless communication techniques.

While OFDM communications have proven useful and indeed beneficial invarious contexts, the present inventors have recognized certaindrawbacks in OFDM. For example, in present OFDM applications, data aretransmitted in a form that is sometimes referred to as an OFDM symbol,which is a collection of parallel data assigned to differentsub-carriers and communicated as a group. Within this OFDM symbol, someof the sub-carriers carry data that is not user data but instead that iscontrol data that describes to the receiver information about the codingand modulation scheme then being used by the transmitter. These controldata are sometimes referred to as training tones or training controldata and part of the information they carry is sometimes referred to asa code parameter set (“CPS”) or generally as transmission parametersignaling. In present OFDM systems, however, the present inventors haveobserved that the CPS information provided by the training tones isrelatively stagnant. Particularly, a typical OFDM system repeatedlycommunicates the same CPS information in every N^(th) successive OFDMsymbols, where N equals three. For example, often a transmitter willmaintain the same CPS information for all operating time betweensuccessive resets. Then, at each reset event, each receiver must be putin some neutral state while the transmitter begins to transmit a new setof CPS information with each N^(th) successively-transmitted OFDMsymbol. Thereafter, each receiver then operates according to the new CPSinformation. Further, because all receivers operate according to thesame CPS information, then typically the transmitter selects the CPSinformation so as to accommodate the weakest communication channelexisting among all of the receivers. As a result, subsequentcommunications to all receivers are based on this worst-case-establishedCPS and, thus, performance with respect to the receivers that couldbenefit from different CPS information are instead constrained by theperformance of the weakest channel. Also as a result of theabove-described manner of communicating CPS information, there islimited flexibility in what may be described by the CPS information. Incontrast, the preferred embodiments seek to increase the scope offlexibility provided by CPS and other control information, which bringsstill other benefits, all of which are discussed in greater detailbelow.

BRIEF SUMMARY OF THE INVENTION

In the preferred embodiment, there is a wireless transmitter. Thetransmitter comprises circuitry for providing a plurality of controlbits and circuitry for providing a plurality of user bits. Thetransmitter further comprises circuitry for modulating the plurality ofcontrol bits and the plurality of user bits into a stream of complexsymbols and circuitry for converting the stream of complex symbols intoa parallel plurality of complex symbol streams. The transmitter furthercomprises circuitry for performing an inverse fast Fourier transform onthe parallel plurality of complex symbol streams to form a parallelplurality of OFDM symbols and circuitry for converting the parallelplurality of OFDM symbols into a serial stream of OFDM symbols. Theserial stream consists of an integer N+1 OFDM symbols. Each OFDM symbolin the serial stream of OFDM symbols comprises a plurality of datapoints. Finally, selected OFDM symbols in the serial stream of OFDMsymbols each comprise control data points carrying a portion of asynchronization code. Other aspects are also disclosed and claimed.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 illustrates a diagram of a wireless communications system by wayof a contemporary orthogonal frequency division multiplexing (“OFDM”)example in which the preferred embodiments operate.

FIG. 2 illustrates an electrical block diagram of transmitter and one ofthe receivers from FIG. 1.

FIG. 3 illustrates a diagram of the data in a first sequence of N+1sequential OFDM symbols designated S1 ₀ through S1 _(N) and ascommunicated according to the prior art.

FIG. 4 illustrates a diagram of the data in a sequence of N+1 sequentialOFDM symbols forming a superframe according to the preferred embodiment.

FIG. 5 illustrates, according to the preferred embodiment, anAMOD-carrying OFDM symbol S_(AMOD) and a later-transmitted OFDM symbolS_(FUT) that is modulated as described by the previously-transmittedOFDM symbol S_(AMOD).

FIG. 6 again illustrates the N+1 sequential OFDM symbols of thesuperframe from FIG. 4, but in this illustration the emphasis is onevery second OFDM symbol in each group of three OFDM symbols in thesuperframe.

FIG. 7 illustrates a functional block diagram of a synchronizationcircuit according to the preferred embodiment.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a diagram of a wireless communications system 10 byway of a contemporary orthogonal frequency division multiplexing(“OFDM”) example in which the preferred embodiments operate. Withinsystem 10 is shown a transmitter TX₁, sometimes referred to as a headend. Transmitter TX₁ is coupled by way of a conductor C_(TX) to atransmit antenna AT_(TX), through which transmitter TX₁ transmits OFDMsignals. Also within system 10 are two receivers RX₁ and RX₂, where forsake of example consider that each receiver is a wireless modem (“WM”)such as in a computer or other computing or data device. Such devicesare sometimes referred to as customer premise equipment (“CPE”). System10 is a fixed wireless system, meaning at least the latter portion ofthe transmission distance (e.g., on the order of a few miles) is fixedby virtue of a fixed location for each receiver; thus, each receiver RX₁and RX₂ is shown by way of example as relating to a respective house H₁and H₂. Each receiver RX₁ and RX₂ is coupled via a respective conductorC_(RX1) and C_(RX2) to a respective receive antenna AT_(RX1) andAT_(RX2). The inclusion of the preferred embodiments in system 10 as afixed wireless system is preferred due to certain attributes of OFDM.Specifically and by way of contrast in mobile environments, there is agreater time-variation in the channel between a transmitter and areceiver, and such a variation diminishes the benefits of theorthogonality in the OFDM communications. Thus, the preferredembodiments have particular benefit in a fixed system, but one skilledin the art also may apply the present inventive teachings in a mobileOFDM system, where preferably such a system also contemplates theadditional complexities arising from the channel variance thataccompanies a mobile system. Lastly, note that FIG. 1 only illustratesthe last distance of transmission in system 10, that is, fromtransmitter TX₁ to receivers RX₁ and RX₂. In actuality, such a systemwill include other aspects not shown in FIG. 1, such as a wirelessaccess termination station (“WATS”) which communicates with a backbonenetwork. Given those additional devices, the WATS transmits totransmitter TX₁, and from these signals transmitter TX₁ transmitsresponsive signals to receivers RX₁ and RX₂.

By way of further introduction to OFDM structure and operation as wellas the improvements implemented by the preferred embodiments, FIG. 2illustrates an electrical block diagram of transmitter TX₁ and one ofthe receivers (e.g., RX₁) from FIG. 1. For the sake of simplifying thediscussion, each of these devices is described separately, below.

With reference to transmitter TX₁, in various respects it resembles theprior art, but in manners discussed below it differs from the prior artand indeed is improved overall due to the manner in which itcommunicates control information to a receiver. Turning then totransmitter TX₁, it receives information bits B_(i) into respective bitformatters 11 a and 11 b. Bits B_(i) may be provided by various knowntypes of circuitry and those bits include both user data (e.g., voice,text, or other user-created data) as well as control data; with respectto the latter, certain of the control data in an OFDM system describesto the receiver information about the coding and modulation scheme thenbeing used by the transmitter. These control data are sometimes referredto as training tones or training control data, the word “training”because it carries information to set up the receiving device whichinitially operates blindly without such information, and the word“control” because the data generally corresponds to the control ofparameters corresponding to the physical layer of communication. Notethat other control data can also be sent over the actual user data whichthen is parsed by a higher layer such as the medium access control(“MAC”). In any event, part of the information carried by the controlinformation is sometimes referred to as a code parameter set (“CPS”). Inthe preferred embodiment, and as detailed later, the scope of thecontrol information and the periodicity at which that informationchanges are provided in inventive manners. In any event, both the userdata and the control data are intended to be included within bits B_(i),where as shown in FIG. 2 the control data is input to bit formatter 11 aand user data is input to bit formatter 11 b. Bit formatter 11 aoperates with respect to the control data as known in the art such as toattach a cyclic redundancy check (“CRC”) mechanism to the bits and toapply any desired format. Bit formatter 11 b orders the user dataaccording to an inventive manner detailed later. Bits B_(i) are outputby bit formatter 11 b to a channel encoder 12. Channel encoder 12encodes the information bits B_(i) in an effort to improve raw bit errorrate, where various encoding techniques may be used. Preferably,therefore, channel encoder 12 performs forward error correction (“FEC”),and in the current embodiment, the channel encoder uses a concatenatedcoding scheme wherein it uses a Reed-Solomon (RS) encoder operating ondata bytes, followed by a convolutional interleaver which interleavesthe data bytes output by the RS encoder which is in turn followed by aconvolutional encoder. In doing the concatenated encoding, the channelencoder 12 uses modulation parameters such as coding rate for theconvolutional encoder, the convolutional interleaver depth, and thenumber of parity bytes added by the RS encoder. It is also possible tochange the channel encoder 12 to use a different coding scheme such as aTurbo encoder in a future version of the embodiment. The appropriateparameters used by the coding scheme are indicated in the signal toreceiver RX₁ by way of control data so that when received by thatreceiver it may use the same parameters to demodulate the complexsymbols for better performance. The encoded output of channel encoder 12is coupled to the input of an interleaver 14 b. Interleaver 14 boperates with respect to a block of encoded bits and shuffles theordering of those bits so that the combination of this operation withthe encoding by channel encoder 12 exploits the time diversity of theinformation. For example, one shuffling technique that may be performedby interleaver 14 b is to receive bits in a matrix fashion such thatbits are received into a matrix in a row-by-row fashion, and then thosebits are output from the matrix to a modulator 16 in a column-by-columnfashion. In the preferred embodiment, interleaver 14 b has a blockmatrix bit interleaving size of 18. Comparably, the output of bitformatter 11 a is connected to an interleaver 14 a, where interleaver 14a operates to interleave the control signal in one of various fashionsthat may differ with respect to the manner in which the user signal isinterleaved by interleaver 14 b. In any event, the output of interleaver14 a is also connected to modulator 16. Modulator 16 is in effect asymbol mapper in that it converts its input bits to complex symbols,each designated generally as s_(i). The converted symbols may takevarious forms, such as quadrature phase shift keying (“QPSK”) symbols,binary phase shift keying (“BPSK”) symbols, or quadrature amplitudemodulation (“QAM”) symbols. Note that modulator 16 might operatedifferently with respect to the user data and the control data. Inaddition, note that modulator 16 may operate on the bits with respect tocertain modulation parameters, where these parameters are detailed laterand are indicated in the signal by way of control data so that whenreceived by a receiver it may use the same parameters to demodulate thecomplex symbols. For example, one such type of parameter is the numberof bits to a symbol mapping. Each symbol s_(i) is coupled to aserial-to-parallel converter 18. In response, serial-to-parallelconverter 18 receives the incoming symbols and outputs n symbols in aparallel stream, along its outputs 18o₁ through 18o_(n), to an inversefast Fourier transform (“IFFT”) block 20. IFFT block 20, as its namesuggests, performs an IFFT on the parallel input data. The data in itsform as output by IFFT block 20 is referred to in the art as an OFDMsymbol, which is not to be confused with each complex symbol s_(i) thatis provided by modulator 16; indeed, the art also uses other terms forthe OFDM symbol such as a burst. For the sake of consistency, in thisdocument the term OFDM symbol is used, and the OFDM symbol as outputfrom IFFT block 20 is connected to a parallel-to-serial converter 22.Parallel-to-serial converter 22 converts its parallel input to a serialoutput. Thus, the OFDM symbol is now a serial stream of information, andthat stream is connected to a cyclic extension block 24. Cyclicextension block 24 adds a prefix and often a postfix to the serialstream of data, where sometimes either or both of these are referred toin the art as a guard interval. The phrase “guard interval” is usedbecause this added period or periods provides an additional guardagainst the effects of multipath delay. Particularly, so long as theguard interval is longer in period than the time between receipt ofdifferent multipaths, then the guard interval in each multipath may beremoved from the data so that the effect of the overlapping receipt ofsignals is removed, thereby greatly improving the probability ofproperly decoding the remaining data. In any event, looking morespecifically to the portions of the guard interval in OFDM, the prefixand postfix are copies of portions of the information from the OFDMsymbol. Specifically, the prefix is a copy of a number of bits from theend of the OFDM symbol, and the postfix is a copy of a number of bitsfrom the beginning of the OFDM symbol. The output of cyclic extensionblock 24 is connected to a radio frequency (“RF”) transmit front end 26,which includes a digital-to-analog converter as well as other analog RFfront-end circuitry. The digital-to-analog converter portion convertsthe digital input to an analog output, and the analog signal isconditioned and passed to transmit antenna AT_(TX) for transmission toreceiver RX₁ (and also receiver RX₂ in the case of FIG. 1).

Turning to receiver RX₁ in FIG. 2, in various respects it resembles theprior art, but in manners discussed below it differs from the prior artand indeed is improved overall due to the manner in which controlinformation is communicated to it from a transmitter and how it usesthat information to decipher sets of data points in the transmittedsignals. Turning then to receiver RX₁, it receives the transmission fromtransmitter TX₁ at its receive antenna AT_(RX1) and the correspondingelectrical signal is connected to an RF receive front end 32. Front end32 includes an analog-to-digital converter as well as other well-knownanalog RF receive front-end circuitry. The analog-to-digital converterportion converts the analog input to a digital output, and the digitalsignal is connected to a synchronization block 34. Synchronization block34 is discussed in greater detail later insofar as it implements aninvention function, but at this point by introduction note that OFDMcommunications require synchronization to the received signals andsynchronization block 34 performs a number of OFDM synchronizations inthe following order: (1) synchronization to the beginning of an OFDMsymbol and removal of the cyclic prefix; (2) synchronization and removalof frequency error between the transmitter and the receiver; and (3)synchronization to the OFDM symbol carrying the CPS information anddecoding of this information. As detailed later, such synchronization isachieved in the prior art by identifying the same set of training tonesas they appear in every third OFDM symbol. However, in the preferredembodiments and also as detailed later, the same training tones are nottransmitted every third OFDM symbol, and, thus an alternativemethodology, such as correlation to a particular code, is implemented bythe preferred embodiments. In any event, once synchronization block 34identifies the location of each OFDM symbol, the result is passed to acyclic extension removal block 36. Specifically, since synchronizationblock 34 has determined the location of the OFDM symbol as received aspart of a transmission, then the prefix, and postfix if one is used, maybe removed since those portions of the transmission are merely copies ofthe data used to overcome the effects of multipath delay. Accordingly,cyclic extension removal block 36 operates in this respect, leaving onlythe OFDM symbol as the remaining information. This remaining informationis output to a serial-to-parallel converter 38. Serial-to-parallelconverter 38 converts its serial input to a parallel output, where thenumber n of outputs corresponds to the same number of n parallel outputsprovided by serial-to-parallel converter 18 of transmitter TX₁. These noutputs from serial-to-parallel converter 38 are connected to a fastFourier transform (“FFT”) block 40. As its name suggests, FFT block 40performs an FFT on the parallel input data, thereby reversing the effectimposed on the information by IFFT block 20 of transmitter TX₁. As aresult, the output of FFT block 40 provides a parallel set of complexsymbols which, assuming proper operation, correspond to and thus are thesame as (or are estimates of) the complex symbols provided by modulator16 of transmitter TX₁. The output of FFT block 40 is connected to ademodulator 44, which removes the modulation imposed on the signal bymodulator 16 of transmitter TX₁. In other words, therefore, whatevertype of symbol mapping was implemented by modulator 16, then demodulator44 performs in effect an inverse of that operation to return the digitalbit data. Moreover, as detailed later, the preferred embodimentsindicate various modulation parameters in the communicated signal fromtransmitter TX₁ to receiver RX₁; thus, these parameters are used bydemodulator 44 to perform its demodulation functionality. The output ofdemodulator 44 is connected to a deinterleaver 46. Deinterleaver 46performs an inverse of the function of interleaver 14 of transmitterTX₁, and the output of deinterleaver 46 is connected to a channeldecoder 48. Channel decoder 48 further decodes the data received at itsinput, typically operating with respect to certain error correctingcodes, and it performs these operations also in part in response tomodulation parameters received from transmitter TX₁ in the control dataof its transmitted signal. Channel decoder 48 outputs a resulting streamof decoded data. Finally, the decoded data output by channel decoder 48may be received and processed by additional circuitry in receiver RX₁,although such circuitry is not shown in FIG. 2 so as to simplify thepresent illustration and discussion.

One aspect according to the preferred embodiment relates to ordering ofbits B_(i) by bit formatter 11 for the transmission of the controlinformation in the OFDM symbols in an inventive manner, and to betterappreciate this aspect attention is now directed in further detail tothe transmission of training tones according to the OFDM prior art.Specifically, FIG. 3 illustrates a diagram of the data in a firstsequence of N+1 sequential OFDM symbols designated S1 ₀ through S1 _(N)and as communicated according to the prior art, where N+1 is a multipleof three. For the sake of simplifying the illustration, it is assumedthat each OFDM symbol is already processed by a receiver through its FFToperation and any guard interval has been removed. As further detailedbelow, each symbol S1 _(x) in the prior art takes one of two forms SF₁or SF₂, where those forms differ in that the control data of form SF₁does not include CPS information while the control data of form SF₂ doesinclude CPS information. Further, a dual-tipped arrow is shown from eachOFDM symbol S1 ₀ through S1 _(N) to demonstrate the form it takes, sothat it may be seen that every third OFDM symbol S1 ₂, S1 ₅, . . . , S1_(N) takes the form SF₂, while the remaining symbols take the form SF₁.With reference to symbol forms SF₁ and SF₂, they are illustrated toinclude a number of vertical arrows intended to illustrate data pointsacross the frequency spectrum encompassed by each symbol; in otherwords, recalling that OFDM communications are along different frequencysub-carriers, then each different vertical arrow within an illustratedOFDM symbol is a data point corresponding to one of the differentfrequency sub-carriers of the OFDM symbol. Thus, the illustration isakin to a frequency spectral plot, as the data would appear after theoperation of FFT block 40 of receiver RX₁. Note that the number of datapoints is shown as 16 only by way of example, when in fact the number ofsub-carriers encompassed by an OFDM symbol may be on the order of 256 or512, or even more sub-carriers.

FIG. 3 is further intended to depict the existence of both control datapoints and user data points in each OFDM symbol. Specifically, certaindata points, at the same sub-carrier location for each OFDM symbol, arecontrol data points, where these control data points are shown usingtaller vertical arrows than the remaining vertical arrows (i.e., datapoints) in each OFDM symbol. The remaining shorter arrows are intendedto illustrate user data points. By way of example, each OFDM symbolincludes four control data points, and the fixed sub-carrier locationfor each such control data point is hereafter referred to as a controldata point sub-carrier. Note also that the choice of four control datapoints is only by way of a simple example when in fact the number ofcontrol points is typically larger for the case when the OFDM symbol has256 (or more) data points. With further reference to the taller verticalarrows indicating control data points, recall that every third OFDMsymbol in the N+1 sequence takes the form SF₂. In the form SF₂, each ofthe control data points other than the first control data point in theOFDM symbol includes a designation of CPS_(X) above the arrow, that is,the three latter control data points are shown to correspond to CPS₁,CPS₂, and CPS₃. These designations are intended to illustrate that eachsuch control data point includes particular CPS information communicatedby that control data point from the transmitter to the receiver. Incontrast, for the remaining OFDM symbols which recall are of the formSF₁, the control data therein does not include nor illustrate any suchCPS information. Note also that the first control data point in eachform SF₂ OFDM symbol does not include CPS information because thatcontrol data point may be unreliable for detection due to filtering inthe receiver.

Having introduced the prior art format of a sequence of OFDM symbols,attention is now directed to certain prior art uses of the control dataincluded in the form SF₁, which recall is in every two out of three OFDMsymbols in a sequence of OFDM symbols. Specifically, the control datathat does not include CPS information represents fixed data, that is,its value as transmitted is fixed and is therefore known to the receiverbefore it is received. As a result, when an OFDM symbol including suchdata is received, the receiver can determine the channel effect based onthe received signal in view of the known transmitted control data. Thisoperation can be shown mathematically, letting y(k) be the receivedsignal, H(k) be the channel effect, and c(k) being the known controldata, as further shown in the following Equation 1:y(k)=H(k)c(k)  Equation 1From Equation 1, since y is received by the receiver and c is known,then mathematically the receiver may solve for H, the channel effect onthe received signal. The channel effect then may be used by the receiverfor other purposes such as later refinement in its estimation of theuser data. Further and as detailed later, the prior art also may use theknown control data for purposes of synchronization.

Attention is now directed to certain prior art uses of the control dataincluded in the form SF₂, which recall is in every third OFDM symbol inan OFDM sequence and includes CPS information. Once a receiver hasreceived the CPS information, it may use that information to decode theuser data points. For example, the CPS may identify the constellationsize of the complex symbols transmitted, such as 4, 16, 32, and soforth. As another example, the CPS may identify a type of forward errorcorrection (“FEC”) to be used by the receiver. As still another example,the CPS may identify the type of interleaver format being implementedwith a transmission. In any event, however, recall that the earlierBackground Of The Invention section of this document discusses variousdrawbacks with the communication of CPS under the prior art, such as therelative stagnant nature of this information because in effect atransmitter routinely transmits the same CPS, as now shown to exist inevery third OFDM symbol, for a considerably lengthy period of time(e.g., maintaining the same information for all time until a next systemreset).

By way of further background, a discussion is now provided as to theprior art manner of determining, for each received OFDM symbol, whetherit takes the form SF₁ or SF₂. In other words, given that a receiverreceives a sequence of OFDM symbols, the receiver must determine if eachOFDM symbol in the sequence is of form SF₁, having only control datapoints without CPS, or of form SF₂, having control data points includingCPS. To determine the form SF₁ or SF₂ of an OFDM symbol, one prior artapproach takes each received OFDM symbol and multiplies each controlpoint sub-carrier in that symbol times the complex conjugate of therespective control point sub-carrier in the immediately preceding OFDMsymbol. For example with reference to FIG. 3, when OFDM symbol S1 _(i)is received following OFDM symbol S1 ₀, then each control pointsub-carrier in S1 ₁ is multiplied times the complex conjugate of therespective control point sub-carrier in S1 ₀. Similarly, when OFDMsymbol S1 ₂ is received following OFDM symbol S1 _(i), then each controlpoint sub-carrier in S1 ₂ is multiplied times the complex conjugate ofthe respective control point sub-carrier in S1 ₁. This process repeatsfor each OFDM symbol, where as detailed below the results of the complexconjugate multiplications indicate whether each OFDM symbol is of theform SF₁ or SF₂.

To further appreciate the effect of the above-described complexconjugate multiplication, consider that each control data point withoutCPS information in an OFDM symbol is the same, and may be represented asin the following Equation 2:e^(j(φ) ^(x) ^(+hφ) ^(x) ⁾  Equation 2In Equation 2, the phase shift term φ_(x) represents the phase componentof the known control data point without CPS information, while the phaseshift term hφ_(x) indicates an additional component in the receivedsignal arising from the channel effect, h, between transmitter TX₁ andreceiver RX₁. Additionally, in Equation 2, any amplitude of the OFDMsymbol is ignored. Thus, for the four control data points in the formSF₁, they may be represented as in the following Equation 3:e^(j(φ) ¹ ^(+hφ) ¹ ⁾, e^(j(φ) ² ^(+hφ) ² ⁾, e^(j(φ) ³ ^(+hφ) ³ ⁾,e^(j(φ) ⁴ ^(+hφ) ⁴ ⁾  equation 3In contrast, the three latter control data points in the form SF₂ haveadded CPS information and, thus, they along with the first control datapoint in the form SF₂ which does not have CPS information, mayrepresented in the following Equation 4:e^(j(φ) ¹ ^(+hφ) ¹ ⁾, e^(j(α) ² ^(+hφ) ² ⁾, e^(j(α) ³ ^(+hφ) ³ ⁾,e^(j(α) ⁴ ^(+hφ) ⁴ ⁾  equation 4Equation 4 mathematically demonstrates the expected result, that is, thecontrol data in form SF₂ differs from form SF₁ due to the inclusion ofthe CPS information in form SF₂ such that the latter three control datapoints in Equation 4 are different than the corresponding Equation 3control data points because the Equation 3 control points do not includeCPS information.

Given Equations 3 and 4, the effects of the earlier-describedcomplex-conjugate multiplication of successive symbol points now may beappreciated. For example, consider first the instance where the controldata points of OFDM symbol S1 ₁ are multiplied time the complexconjugate of the respective control data points of OFDM symbol S1 ₀.Since both OFDM symbols S1 ₀ and S1 ₁ are of the form SF₁ as shown inEquation 3, then each such multiplication yields the product of acomplex number times its own complex conjugate, assuming the channeleffect, h, is relatively constant from the time between receipt of S1 ₀and S1 ₁, as it most often is in a fixed system; one skilled in the artwill appreciate that each such respective multiplication yields only areal number (relating to the multiplicand's amplitude) because anyimaginary component is removed by such a multiplication. In contrast,consider second the instance where the control data points of OFDMsymbol S1 ₂ are multiplied time the complex conjugate of the respectivecontrol data points of OFDM symbol S1 ₁, and for sake of simplificationconsider all the control data points other than the first one in eachsymbol so as to ignore the effect that the form SF₂ does not include CPSinformation in its first control data point. Thus, since the controldata points in OFDM symbol S1 ₂, as shown in Equation 4, differ from S1₁, as shown in Equation 3, (because the former includes CPSinformation), then each respective multiplication yields a result with anon-negligible phase term (i.e., an imaginary component). Similarly,consider third the instance where the control data points of OFDM symbolS1 ₃ (e.g., Equation 3) are multiplied time the complex conjugate of therespective control data points of OFDM symbol S1 ₂ (e.g., Equation 4);once more the control data points differ because S1 ₃ does not includeCPS information while S1 ₂ does, and again therefore then eachrespective multiplication yields a result with a non-negligible phaseterm. Lastly, consider fourth the instance where the control data pointsof OFDM symbol S1 ₄ (e.g., Equation 3) are multiplied time the complexconjugate of the respective control data points of OFDM symbol S1 ₃(e.g., also Equation 3). Since both OFDM symbols S1 ₄ and S1 ₃ are ofthe form SF₁, then each respective multiplication yields the product ofa complex number times its own complex conjugate, leaving a product withonly a real number. Given the preceding, the prior art sums the resultsof each multiplication of each respective control point pair for a pairof compared OFDM symbols. Thus, such a sum will be at a maximum when twosuccessive form SF₁ symbols are compared by the above-describedmultiplication process, because that sum will include only the realvalues realized from each multiplication; conversely, the sum will besmaller for any multiplication between a form SF₁ and form SF₂ symbolsince the products of those operations will include non-negligibleimaginary terms. As a result, therefore, for each series of threesuccessive OFDM symbols, the timing of a determined maximum will therebycorrespond to an incident where two form SF₁ symbols have been received,and from that knowledge it is further known from the format of FIG. 3that the next OFDM symbol (i.e., following the two detected form SF₁symbols) will be a form SF₂ symbol. Given that knowledge, it is furtherknown that the control data points in that next OFDM symbol (i.e., ofform SF₂) will include CPS information and, therefore, that next OFDMsymbol may be further analyzed to detect the CPS information as suchdetection is further detailed below. Thereafter, the use of the CPSinformation may be for any of the various purposes described above. Inaddition, the knowledge presented by the above-described peaks alsoserve to identify the location of the beginning of each received OFDMsymbol.

Having demonstrated how the prior art determines whether each OFDMsymbol is of the form SF₁ or SF₂, attention is now directed to thedetection of the CPS information according to the prior art, which alsoprovides further background for the later discussion of the preferredembodiments. First, recall that the CPS information is embodied in eachterm α_(x) in Equation 4. Second, since each value φ_(x) is known, thenone manner of determining each value α_(x) would be first to solve forthe channel effect, h, and then solving for the remaining unknown termα_(x) in Equation 4. For example, the channel effect, h, could bedetermined by multiplying each term in Equation 3 times the respectiveitem in the following Equation 5:e^(−j(φ) ¹ ⁾, e^(−j(φ) ² ⁾, e^(−j(φ) ³ ⁾, e^(−j(φ) ⁴ ⁾  Equation 5Such a multiplication would yield the results shown in the followingEquation 6:e^(j(hφ) ¹ ⁾, e^(j(hφ) ² ⁾, e^(j(hφ) ³ ⁾, e^(j(hφ) ⁴ ⁾  Equation 6Thus, in Equation 6, each value φ_(x) is known, so the remaining value hmay be determined. Once h is determined in such a manner, it may be usedwith Equation 4 to determine each value of α_(x), that is, to determinethe CPS information.

While the preceding description of detecting the CPS information inα_(x) represents a workable approach, the prior art has provided analternative that requires fewer computations. Specifically, in the art,it is known to make the substitution shown in Equation 7 for α_(x) inEquation 4:α_(x) =CPS _(x+φ) _(x)  Equation 7For example, by substituting the definition of α_(x) from Equation 7into the elements of Equation 4, then the following Equation 8 isrealized for the form SF₂ OFDM symbol:e^(j(α) ¹ ^(+hφ) ¹ ⁾=e^(j(CPS) ¹ ^(+φ) ¹ ^(hφ) ¹ ⁾, e^(j(CPS) ² ^(+φ) ²^(hφ) ² ⁾, e^(j(CPS) ³ ^(+φ) ³ ^(+hφ) ³ ⁾, e^(j(CPS) ⁴ ^(+φ) ⁴ ^(hφ) ⁴⁾  Equation 8Now, recall from above that to locate the forms SF₁ and SF₂ within astream of OFDM symbols, multiplication is performed between the controldata points in an OFDM symbol with the complex conjugates of therespective control data points in the immediately-previous OFDM symbol.Thus, when a form SF₁ OFDM symbol is received immediately followinganother form SF₁ OFDM symbol, then the result is the same as describedearlier, that is, the phase terms are removed and a maximum amplitude isachieved. However, when an SF₂ OFDM symbol is received, and recallingthat from the pattern of FIG. 3 that such an OFDM symbol follows an SF₁OFDM symbol, then the multiplication, with respect to respective controldata points, of that SF₂ OFDM symbol times the complex conjugate of theimmediately-preceding SF₁ OFDM symbol, realizes the following Equation9:e ^(−j(φ) ^(x) ^(+hφ) ^(x) ⁾ ×e ^(j(CPS) ^(x) ^(+φ) ^(x) ^(+hφ) ^(x) ⁾=e ^(j(CPS) ^(x) ⁾  Equation 9From Equation 9, it may be seen that the terms φ_(x) and hφ_(x) cancelout, leaving only the phase indication of CPS_(x). In other words, dueto the substitution of Equation 7, then the same multiplication processthat identifies the location of each SF₂ OFDM symbol at the same timealso produces the value of CPS_(x) for each such OFDM symbol. Thus, theCPS information results from the same complex conjugate multiplicationstep used to identify the locations of the SF₁ and SF₂ form OFDM symbolsand, hence, there is no need to separately solve for h as describedabove with respect to Equations 5 and 6.

Turning now to additional aspects of the preferred embodiments, FIG. 4illustrates a diagram of the data transmitted and received in aninventive sequence of N+1 sequential OFDM symbols designated S2 ₀through S2 _(N), where for reasons described below N=179 in thepreferred embodiment (i.e., the stream includes N+1=180 OFDM symbols)and where the form of superframe SPRF is provided at least in part byformatter 11 in FIG. 2. As in the case of FIG. 3, but here with respectto the preferred embodiments, assume for the sake of simplifying theillustration that each OFDM symbol is represented in the frequencydomain and without a guard interval, that is, either as it would appearat the output of IFFT 20 prior to transmission or at the output of FFTblock 40 after being received and partially processed. For the sake ofreference and as processed according to the preferred embodiment, thesequence of 180 OFDM symbols in FIG. 4 is referred to as a superframeSPRF. In the preferred embodiments and for reasons discussed later,superframe SPRF has a number of OFDM symbols equal to an integermultiple of three and also equal to an integer multiple of the bitinterleaving size of interleaver 14; recall from FIG. 2 that interleaver14 in the preferred embodiment has an interleaving size of 18 and, thus,in FIG. 4 superframe SPRF has a number of OFDM symbols equal to theinteger 10 times this interleaving capacity. In the preferredembodiment, for every group of three OFDM symbols, and with respect tothe absence of CPS information, the first two OFDM symbols take a formSF₁′ which is comparable in various respects to form SF₁ of the priorart. Specifically, in the preferred embodiment, the form SF₁′ OFDMsymbols include user data points and evenly-spaced control data pointsshown by shorter and longer vertical arrows, respectively, where thecontrol data points do not include CPS information. As detailed later,however, in the preferred embodiment each second of the first two SF₁′OFDM symbols in a group of three OFDM symbols includes additionalinformation for purposes of synchronization. Looking now to every thirdOFDM symbol of superframe SPRF, it has a form that is comparable in somerespects to form SF₂ but also that provides additional diversity. Theseaspects are detailed below.

To further understand every third OFDM symbol in superframe SPRF,attention is first turned by way of example to OFDM symbol S2 ₂.Specifically, as shown by a dual-tipped arrow in FIG. 4, OFDM symbol S2₂ takes a form SF_(2.1). At first glance, form SF_(2.1) is comparable toform SF₂ of FIG. 3, but note that form SF_(2.1) preferably includesinformation that differs from the prior art CPS information.Specifically, form SF_(2.1) includes both user data points shown assmaller vertical arrows and evenly-spaced control data points shown astaller vertical arrows. However, with respect to all control data pointsother than the first control point, they include what is referred toherein as adaptive modulation (“AMOD”) information, which includes CPSinformation, but as its name suggests, which also permits changes (i.e.,it is adaptive) in modulation parameters across different superframes asfurther detailed later. Accordingly, in this document, each OFDM symbolof the form SF_(2.1) may be referred to as an AMOD-carrying OFDM symbol.In the preferred embodiment, the AMOD information is modulated on thecontrol data points in the same manner as described above with respectto the CPS information and in connection with Equations 7 and 8 and,thus, for the AMOD information Equation 7 may be re-written as thefollowing Equation 10:α_(x) =AMOD _(x)+φ_(x)  Equation 10In addition, the AMOD information may include information beyond CPSinformation, as also detailed later. At this point and for the sake ofillustrating the AMOD information, each taller vertical arrow (i.e.,control point) in form SF_(2.1), other than the first taller arrow,points to a corresponding label of AMOD_(1.1), AMOD_(2.1), andAMOD_(3.1), where the subscript number preceding the decimal in eachsuch designation is intended to indicate a different AMOD value percontrol point, and where the subscript number following the decimal ineach such designation is intended to indicate that the AMOD informationcorresponds to the form SF_(2.1) symbol in the superframe. As shownbelow, for other values of x having a corresponding form SF_(2.x),different values of AMOD information AMOD_(1.x), AMOD_(2.x), andAMOD_(3.x), are provided by the corresponding AMOD-carrying OFDM symbol,and these other OFDM symbols are also referred to as AMOD-carrying OFDMsymbols. In any event, therefore, given the forms SF_(2.1), SF_(2.2),and SF_(2.3), for each successive AMOD-carrying OFDM symbol insuperframe SPRF, it carries different AMOD information than theimmediately preceding AMOD-carrying OFDM symbol. Lastly, note thatpresently in the preferred embodiment, each AMOD-carrying control pointmay carry two bits of AMOD information.

Looking further now to the contrast of the preferred embodiment with theprior art and continuing with FIG. 4, attention is directed to OFDMsymbol S2 ₅, that is, the second AMOD-carrying OFDM symbol in superframeSPRF. As now shown, the AMOD information in AMOD-carrying OFDM symbol S2₅ differs from the AMOD information in the immediately precedingAMOD-carrying OFDM symbol, namely, S2 ₂. Specifically, OFDM symbol S2 ₅takes a form SF_(2.2) which represents a comparable format to formSF_(2.1). However, the subscript number of the form SF₂₂ following thedecimal indicates a value of 2 to correspond to the AMOD informationAMOD_(1.2), AMOD_(2.2), and AMOD_(3.2) that is included withinrespective control data points of OFDM symbol S2 ₅. Thus, this AMODinformation differs from the AMOD information AMOD_(1.1), AMOD_(2.1),and AMOD_(3.1) that is included in form SF_(2.1) of AMOD-carrying OFDMsymbol S2 ₂. Similarly, with reference to the third AMOD-carrying OFDMsymbol in superframe SPRF (i.e., OFDM symbol S2 ₈), it takes the formSF_(2.3) which includes AMOD information AMOD_(1.3), AMOD_(2.3), andAMOD_(3.3), which therefore differs from the AMOD of both OFDM symbol S2₂ having the form SF_(2.1) and OFDM symbol S2 ₅ having the formSF_(2.2).

For the sake of the remaining discussion in this document, and havingestablished that the AMOD information in forms SF_(2.1), SF_(2.2) andSF_(2.3) differs, then those forms when combined may be thought of asforming a single AMOD message. As further shown below, this AMOD messagemay contain various parameters. Also in the preferred embodiment and asdetailed later, the AMOD message includes multiple groups of AMODinformation, where each AMOD group includes the same types of parametersand those parameters apply to a set of data points identified by thegroup, but the values of those parameters may differ for each differentAMOD group. From this format and also as detailed later, each differentAMOD group may correspond to a different set of data points, so that byway of example a first AMOD group provides the modulation informationcorresponding to how a first set of control points were modulated whentransmitted (and how they therefore can be demodulated upon receipt),while a second AMOD group provides the modulation informationcorresponding to how a second set of control points were modulated whentransmitted (and how they therefore can be demodulated upon receipt),and possible additional AMOD groups in the same manner. As a result, inthe preferred embodiment, each AMOD message, as provided in a singlesuperframe, may include multiple AMOD groups, each corresponding to adifferent set of corresponding data points.

Completing FIG. 4 and looking to the last three OFDM symbols insuperframe SPRF, they again represent a sequence of three OFDM symbols,where the first two of the three OFDM symbols, namely, S2 ₁₇₇ and S2₁₇₈, take the form SF₁′. The last OFDM symbol in that group of OFDMsymbols, namely, S2 ₁₇₉, does not take the form SF₁′ because it alsoincludes AMOD information in all but the first of its control points. Inone embodiment, such AMOD information may differ from the AMODinformation in all of the preceding AMOD-carrying OFDM symbols insuperframe SPFR (i.e., S2 ₂, S2 ₅, S2 ₈, . . . S2 ₁₇₆). In this case, asingle AMOD message is provided by all 60 of the AMOD-carrying OFDMsymbols in superframe SPRF. However, in the preferred embodiment, atsome point the AMOD message carried by multiple AMOD-carrying OFDMsymbols in superframe SPFR is repeated within that same superframe SPFR.In other words, the AMOD message is formed by numerous AMOD-carryingOFDM symbols in the superframe, but it is completed in less than theentirety of the 60 AMOD-carrying OFDM symbols in the superframe. Forexample, consider an instance where each OFDM symbol includes 16 controldata points, so all 15 of the latter control data points include AMODinformation; assume further that the entirety of the AMOD message soughtto be included in superframe SPFR can be expressed in a total of 45control data points. As a result, since three successive AMOD-carryingOFDM symbols can provide 45 control data points (i.e., 15 control datapoints/symbol*3 symbols=45) worth of AMOD information, then once threesuch AMOD-carrying OFDM symbols are provided (which occurs after a totalof nine OFDM symbols pass since every third symbol is an AMOD-carryingsymbol), then the AMOD message is complete as of that time. Accordingly,in the preferred embodiment, the AMOD message is then repeated startingwith the next AMOD-carrying OFDM symbol. In other words, by applyingthis example to FIG. 4, then AMOD-carrying OFDM symbols S2 ₂, S2 ₅, andS2 ₈ would together provide the desired AMOD message, and the nextAMOD-carrying OFDM symbol S2 ₁₁ would carry the same AMOD information asOFDM symbol S2 ₂, that is, OFDM symbol S2 ₁₁ would take the formSF_(2.1). Continuing this example, OFDM symbol S2 ₁₄ would take the formSF_(2.2), and OFDM symbol S2 ₁₇ would take the form SF_(2.3); thus, theentirety of OFDM symbols S2 ₁₁, S2 ₁₄, and S2 ₁₇ would be a repetitionof the same AMOD message in OFDM symbols S2 ₂, S2 ₅, and S2 ₈. FIG. 4also completes this example with respect to OFDM symbol S2 ₁₇₉, namely,because it is the third AMOD-carrying OFDM symbol in the sequence of S2₁₇₃, S2 ₁₇₆, and S2 ₁₇₉, then this sequence again repeats the AMODmessage wherein OFDM symbol S2 ₁₇₃ takes the form SF_(2.1), OFDM symbolS2 ₁₇₆ takes the form SF_(2.2), and OFDM symbol S2 ₁₇₉ takes the formSF_(2.3). Naturally, if the AMOD message requires more or less thanthree AMOD-carrying OFD symbols to complete within superframe SPFR or ifa different amount of AMOD information may be communicated perAMOD-carrying OFDM symbol, then the number of forms SF_(2.x) areadjusted accordingly.

Given the preceding discussion of FIG. 4, one skilled in the art shouldappreciate that for a collection of OFDM symbols according to thepreferred embodiment, such as in superframe SPFR, they include a samenumber of control data points as the prior art, but in that same numberof data points a much greater amount of AMOD information may be carriedas opposed to the CPS information carried by the prior art. Accordingly,in the preferred embodiment the AMOD information varies as between everythird OFDM symbol, whereas in the prior art generally every third OFDMsymbol in a sequence of numerous OFDM symbols included the same CPSinformation. Indeed, recall it is stated much earlier that the prior artmight maintain the same CPS information between successive resets. Thus,in the prior art, thousands and even millions of successive OFDM symbolsmay be communicated between a transmitter and receiver where each thirdOFDM symbol communicates the same CPS information. In contrast, thepreferred embodiment can vary its AMOD information as between successiveAMOD-carrying OFDM symbols. Thus, the preferred embodiment contemplateschanges in the control information (i.e., the AMOD) between every thirdOFDM symbol whereas the prior art repeats the same control information(i.e., the CPS) for lengthy periods of time. Given this capability ofthe preferred embodiment, one further aspect that is preferred is todefine sets of data points within the OFDM symbols and to have each suchset correspond to a different group of AMOD information; this aspect isillustrated in connection with FIG. 5, as described below.

FIG. 5 illustrates two OFDM symbols in a sequence, with it understoodthat various symbols exist between the two illustrated OFDM symbols inthe same sequence; however, for the sake of simplification, suchadditional OFDM symbols are not shown. In the preferred embodiment, thetwo illustrated OFDM symbols are in different superframes, for reasonsmore clear later. In any event, looking to FIG. 5, a first OFDM symbolin FIG. 5 is an AMOD-carrying OFDM symbol S_(AMOD) according to thepreferred embodiment and again is shown in the frequency domain andwithout a guard interval like FIGS. 3 and 4. Symbol S_(AMOD) representsthe form SF_(2.x) from FIG. 4, with greater elaboration in theillustration of FIG. 5 and the following discussion. Symbol S_(AMOD)includes 256 data points, which may be of three different kinds. A firstkind of data point is a user data point, as discussed earlier. A secondkind of data point is a so-called zero data point, which represents asub-carrier that does not carry either user or control information. Boththe first and second kinds of data points are shown in FIG. 5 usingrelatively short vertical arrows. A third kind of data point, shownusing taller vertical arrows, is an AMOD-carrying control data point,which as introduced earlier is included in a form SF_(2.x) OFDM symbol.Thus, symbol S_(AMOD) includes control data points that include AMODinformation such as in forms SF_(2.1), SF_(2.2), and SF_(2.3) in FIG. 4.To better illustrate an additional aspect of the preferred embodiment,rather than the simplified example of only four control data points asin FIG. 4, FIG. 5 illustrates a more realistic example wherein symbolS_(AMOD) includes 16 evenly-spaced control data points indicated bytaller vertical arrows; moreover, all but the first control data pointin symbol S_(AMOD) includes AMOD information, so each of the 15AMOD-carrying control points is shown with a corresponding piece of theAMOD information AMOD_(1.1) through AMOD_(15.1). Further, following eachcontrol data point is 15 user data points. Accordingly, symbol S_(AMOD)includes a total of 256 data points. Due to the limited space in FIG. 5,however, only the first 32 data points and the last 16 data points areshown in their entirety, with the remaining of the 256 data points to beunderstood to also exist in symbol S_(AMOD).

By way of introduction to one of the specific types of communicationinformation that are included in the preferred embodiment AMODinformation of symbol S_(AMOD), FIG. 5 further includes a symbolS^(FUT), which includes the “FUT” subscript to suggest that symbolS_(FUT) is transmitted and received at some future time relative tosymbol S_(AMOD), that is, in a stream of OFDM symbols over time, S^(FUT)is subsequent to S_(AMOD). Symbol S_(FUT) also includes a total of 256data points, including 16 evenly-spaced control data points with 15other data points (either user or zero) following each of theevenly-spaced control data points. For reasons more clear below, thefirst 128 of the 256 data points of symbol S_(FUT) are shown in expandedfashion as are the last 16 of the data points in symbol S_(FUT).

Returning to symbol S_(AMOD) in FIG. 5, in the preferred embodiment, theentirety of its AMOD information, as represented by AMOD_(1.1) throughAMOD_(15.1), is used, either alone or in combination with AMODinformation from additional AMOD-carrying symbols as described later, toprovide an AMOD message to describe how information is modulated in afuture OFDM symbol such as in symbol S_(FUT). As a result, receiver RX₁receives symbol S_(AMOD) and decodes its AMOD information, and thenlater uses that information to demodulate the data points in thelater-received symbol S_(FUT). Further toward this end, the AMODinformation in symbol S_(AMOD) includes a parameter referred to hereinas set_size. The parameter set_size is therefore part of the AMODmessage and defines a set consisting of a number of data points in afuture OFDM symbol (e.g., S_(FUT)), where that number of user datapoints are modulated according to additional parameters set forth in theAMOD information. By way of example, therefore, S_(AMOD) carries AMODinformation which includes various types of modulation informationdescribed later and also a value of set_size, and set_size defines anumber of data points in S_(FUT) which are modulated by the othervarious AMOD information specified in S_(AMOD). In the preferredembodiment, the number of bits in set_size may be different values,depending on the maximum number of data points anticipated to be definedin a set. For example, in the case of FIG. 5, let the maximum number ofmaximum number of data points anticipated to be defined in a set beequal to 256 data points, that is, S_(AMOD) may specify up to 256 datapoints in S_(FUT) to be demodulated according to the AMOD information inS_(AMOD). As a result, set_size will be eight bits. Given a range ofeight bits, set_size may be equal to any number up to 256. A firstexample EX1 is shown in FIG. 5, where set_size=16; in this case then thefirst 16 data points in S_(FUT) are modulated according to other AMODinformation provided by S_(AMOD). A second example EX2 is also shown inFIG. 5, where set_size=32; in this case then the first 32 data points inS_(FUT) are modulated according to other AMOD information provided byS_(AMOD). FIG. 5 further illustrates numerous other examples, where allthe examples of FIG. 5 are set forth in the following Table 1:

TABLE 1 set_size example 16 EX1 32 EX2 48 EX3 128 EX4

Having demonstrated in FIG. 5 the use of set_size, further elaborationis helpful in connection with the earlier statement that in thepreferred embodiment the AMOD message in superframe SPRF includesmultiple groups of AMOD information, where each group corresponds to adifferent set of data points. For example, assume that a first AMODgroup G1 in an AMOD message has its parameter set_size equal to 48, asin the case of EX3 in Table 1 and FIG. 5. As a result, therefore, theremaining modulation information in group G1 characterizes themodulation of those 48 data points in S_(FUT). However, given thisexample, this leaves the latter 80 of the 128 data points of S_(FUT) tobe described by different AMOD data. To complete the example, therefore,assume also that the AMOD message from the superframe that includeS_(AMOD) includes two other AMOD groups G2 and G3, wherein AMOD group G2has its parameter set_size equal to 16 and wherein group AMOD G3 has itsparameter set_size equal to 64. As a result, after the 48 data points ofS_(FUT) shown in FIG. 5 as EX3, the next 16 data points of S_(FUT) aremodulated according to the AMOD information in AMOD group G2, while thefinal 64 data points of S_(FUT) are modulated according to the AMODinformation in AMOD group G3.

Two additional observations are also noteworthy with respect to theparameter set_size as further demonstrated through the previous examplesand Table 1. First, while the examples of Table 1 illustrate set_sizeequal to values that are a multiple of 16, other values where this isnot the case may be used. Second, FIG. 5 simplifies the illustration ofthe collection of data points in sets by depicting the sets ascollections of data points in the frequency domain starting from left toright, that is, in a linear fashion. However, in the preferredembodiment the actual transmission is not in linear order, but insteadthe user and control data points are scrambled in a certain fashion,such as through the use of bit reversal or some other technique tospread out the information and, thus, decrease the effect of anycommunication errors. Bit reversal is known in the art and generallyinvolves taking a binary bit assignment value and reversing the order ofthose binary bits to change the assignment. For example, if the decimalvalue 1 is indicated as a seven bit number (i.e., 0000001), then inbit-reversed fashion it indicates a value of decimal 64 (i.e., 1000000).Thus, if a data point corresponds to a linear location of 1, then in bitreversed fashion it instead is mapped to location of 64. In addition, inthe preferred embodiment, the AMOD-carrying control data points are infixed locations, and the zero data points are split equally at thebeginning and end of each OFDM symbol. For example, assume that an OFDMsymbol includes a total of 128 data points, wherein 90 of those datapoints are user data points, 16 of those data points are AMOD-carryingcontrol data points, and the remaining 22 of the data points are zerodata points. In such a case, each of the 16 AMOD-carrying control datapoints are at a fixed location, such as every eighth data point, and the22 zero data points are split equally with 11 zero data points at thebeginning of the OFDM symbol (other than in each eighth location whichis reserved for an AMOD-carrying control data point) and 11 zero datapoints at the end of the OFDM symbol. The remaining data points, notoccupied by either an AMOD-carrying control data point or a zero datapoint, are the locations for the user data points. However, with bitreversal, when the data points are mapped to bit-reversed locations,some of the bit reversals may identify a location already occupied by anAMOD-carrying data point (e.g., every eighth location) or a zero datapoint; preferably, the location of the AMOD-carrying data point and thezero data point is maintained and the bit-reversed location for the datapoint is instead further re-assigned so that the AMOD-carrying datapoint or the zero data point can remain in its fixed location. Giventhis example or any other type of scrambling, as the information isreceived in bit reversed order, thereafter a bit reversal or otherappropriate de-scrambling is performed so that the symbols adjacent toeach other in bit-reversed fashion will be returned to their originalorder. Given this bit reversal formation or other scrambling technique,when set_size defines a number of data points in a set, in the preferredembodiment the set is defined in the bit reversed or scrambled formatrather than the more simplified version as shown in FIG. 5. In otherwords, the data points in each set defined by a corresponding value ofset_size may be mapped in various fashions, and this mapping is alsotaken into account so that set_size identifies the data points in thatset and as scrambled across S_(FUT) rather than as the data points wouldappear in linear fashion.

With the above having demonstrated that in the preferred embodiment eachAMOD group defines a set of data points in a future OFDM symbol that ismodulated according to corresponding AMOD information, an additionalAMOD parameter is now described and is included in each AMOD group.Specifically, each AMOD group also includes an indication of a numberassigned to the superframe in which the present AMODinformation-carrying OFDM symbol is located, and this number isdesignated herein as SPRF_no. For example in FIG. 5, the AMOD-carryingsymbol S_(AMOD) has a value of SPRF_no. that identifies the superframein which S_(AMOD) is included. In the preferred embodiment, SPRF_no is a4 bit number and, thus, may specify a value from 0 to 15; thus, a totalof up to 16 different superframe numbers may be specified, andpreferably therefore the superframe number increments for eachsuperframe and is a value of modulo 16.

Another aspect of the preferred embodiment is directed to the timing ofwhen AMOD parameters may change, and this aspect is also understood withreference to an additional AMOD parameters specified in each AMOD groupas well as the previous discussion of FIG. 5. Particularly, recall fromFIG. 5 that the AMOD information in one AMOD-carrying OFDM symbol (e.g.,S_(AMOD)) defines the modulation parameters for a future OFDM symbol(e.g., S_(FUT)). In the preferred embodiment, the location of the futureOFDM symbol is in a superframe transmitted and received later than thepresent AMOD-carrying OFDM symbol. Specifically, recall that superframeSPRF includes a number of OFDM symbols, where in the illustrated examplethat number equals 180. As a result, a given superframe has a boundarythat separates it from an immediately-following successive superframe,and so forth for future-transmitted superframes. Given the grouping ofOFDM symbols into such a format, in the preferred embodiment, AMODinformation may be changed at any superframe boundary. In other words,because each superframe is a multiple of the interleaving size (e.g., amultiple of 18), then the bit interleaver returns to the same phase atthe start of every superframe. Thus, when AMOD information is to bechanged, then each AMOD group in an AMOD message in a given superframeindicates a future superframe in which different modulation is used forthe data point set corresponding to each group. Particularly, each AMODgroup also includes a superframe number for a superframe in the futureto which the present AMOD group will apply, and this number isdesignated herein as fut_SPRF_no. For example, suppose that receiver RX₁is presently processing a superframe having a value of SPRF_no=3according to AMOD information specified in the present superframe. Inthe preferred embodiment, such AMOD information is not for applicationto the present superframe (i.e., SPRF_no=3), but instead such AMODinformation is to be applied to a subsequent superframe, such as thenext superframe which in the present example would be SPRF_no=4 (or at alater superframe having SPRF_no>4); in this example, therefore, the AMODinformation in the present superframe will include a value offut_SPRF_no=4. Accordingly, when the superframe number 4 is laterreceived (as detected by a method described later), then the AMODmessage from SPRF_no=3 is the proper modulation information fordeciphering the data points in superframe SPRF_no=4. Thus, modulation aswell as the characterization of that modulation by the AMOD informationmay change as between different superframes. Moreover, recall that eachsuperframe SPRF is preferably an integer multiple of the bitinterleaving size of interleaver 14 (e.g., 18). As a result, changes inthe AMOD do not require any re-synchronization of the forward errorcorrection, and similarly therefore there is no need to discontinuedownstream communications in order to change AMOD values as is requiredwith respect to CPS in the prior art.

According to the preferred embodiment, each AMOD group also includes anindication of the version of the broadband wireless internet forum(“BWIF”), and this indication is designated herein as BWIF_type.Specifically, BWIF is presently a program of the IEEE Industry andStandards and Technology Organization that is directed toward solutionsinvolving OFDM systems. Presently, the BWIF has provided a version 1.0set of specifications, and later versions are reasonably to beanticipated. Accordingly, because the preferred embodiment permits thecommunication of considerably more control information as compared tothe prior art, then the preferred embodiment is also more easilyadaptable to newer BWIF versions as specified by BWIF_type, oralternatively the preferred embodiments are therefore in effectprogrammable so that some data points can be modulated and detectedaccording to one BWIF specification and in response to a first value ofBWIF_type, while others data points can be modulated and detectedaccording to a different BWIF specification in response to a differentvalue of BWIF_type. In another approach, BWIF_type specifies the BWIFstandard version compatibility of the headend or the WATS to which theWM is connected. Indeed, note further that the coding parameters,discussed below, can have different meanings based on thecorrespondingly-specified BWIF_type.

Each AMOD group in the preferred embodiment also carries, as introducedearlier, CPS information, where CPS information is known in the art.However, the preceding has shown that this CPS information, as part ofan AMOD group, can be different in one group versus another group orgroups, whereas the prior art uses the same CPS across numeroussuccessive OFDM symbols and the data points therein. In any event, withrespect to each AMOD group, the CPS information includes generally fourdifferent parameters: (i) modulation type; (ii) coding rate; (iii)number of RS parity bits; and (iv) interleaver depth. Note that items(ii) through (iv) are sometimes referred to in the art as forward errorcorrection (“FEC”) parameters. Looking to each of the four CPSparameters, modulation type relates to the type of symbol mappingimplemented by modulator 16, and may be specified using two bits. In thepreferred embodiment, presently included would be an indication of froma set of 4, 16, 64, and 256 QAM, but as mentioned earlier, other symbolmapping modulation may be used and, hence, the modulation type includedin the AMOD information may accommodate these alternatives as well.Coding rate is an indication of the specific rate of the convolutionalencoder in the channel encoder block 12, and is presently chosen fromone of

$\frac{1}{2},\frac{2}{3},\frac{3}{4},\frac{5}{6},{{and}\mspace{14mu}\frac{7}{8}},$andmay be specified using three bits. Number of RS parity bits describesthe number of parity bytes added by the RS encoder in the channelencoder block 12 and is presently chosen to be either 20 or 22 bytes,and with only two choices may be specified using one bit. Finally, theinterleaver depth specifies the number of branches or the depth of theconvolutional interleaver, the actual number is chosen from a fixed setdepending on the FFT length.

Finally, the present inventors further contemplate that the scope ofparameters included in the AMOD information certainly may be expanded,particularly given the great deal of additional control data points thatare able to carry different AMOD information under the preferredembodiment. As another example, each AMOD group may be used to specify adifferent type of coding (e.g., turbo coding). Indeed, with other typesof coding, the AMOD information also may specify parameters for thatcoding type. Other examples may be ascertained by one skilled in theart. Another type of information preferably included in each AMOD groupis a cyclic redundancy check (“CRC”) indication, as may be specifiedusing eight bits. The CRC is used to check if the AMOD parameters havebeen received without errors.

Another aspect of the preferred embodiment is directed to detecting theboundary between each received superframe. Such synchronization may berequired for various reasons, and has particular application inconnection with the earlier discussion of fut_SPRF_no in the AMODinformation of a present superframe in order to specify AMOD informationfor applying to a future-received superframe. In other words, in orderto apply AMOD information to a future-received superframe, there isnecessarily a need to indentify the boundary of when thatfuture-received superframe is received. This aspect is now describedwith reference to FIGS. 6 and 7, as detailed below.

FIG. 6 again illustrates the N+1 sequential OFDM symbols of superframeSPRF from FIG. 4 and as may be presented in response to the formattingof formatter 11 in FIG. 2, but the FIG. 6 illustration emphasizes everysecond OFDM symbol in each group of three OFDM symbols in superframeSPRF. Specifically, recall in connection with FIG. 4 that it is statedthat for every group of three OFDM symbols, the first two OFDM symbolstake a form SF₁′ which include user and evenly-spaced control datapoints, where the control data points do not include CPS information. Asillustrated in FIG. 6, however, the form of each second OFDM symbol ofthese groups of OFDM symbols does differ from the first OFDM symbol, notin the manner of CPS information, but in the manner of a synchronizationcode. Thus, each second OFDM symbol of these groups of OFDM symbols isshown to have a different form SF_(1.z)′, where the difference in eachsuch form as demonstrated by a change in z is as detailed below.

In the preferred embodiment, in each group of three OFDM symbols, thecontrol points in each second OFDM symbol (other than the first controlpoint in each such OFDM symbol) includes a different portion of a60-digit synchronization code. For illustration, let the code bedesignated as shown in the following Equation 11:synchronization code={C₁, C₂, C₃, . . . C₆₀}  Equation 11

Given Equation 11, each different code digit C_(y) is carried by arespective second OFDM symbol in a group of three OFDM symbols. Forexample, digit C₁ is carried by OFDM symbol S2 ₁, which is the secondOFDM symbol in the first group of three OFDM symbols and which thereforeis shown to have the form SF_(1.1)′. As another example, digit C₂ iscarried by OFDM symbol S2 ₄, which is the second OFDM symbol in thesecond group of three OFDM symbols and which therefore is shown to havethe form SF_(1.2)′. As another example, digit C₃ is carried by OFDMsymbol S2 ₇, which is the second OFDM symbol in the third group of threeOFDM symbols and which therefore is shown to have the form SF_(1.3)′. Asa final example, digit C₆₀ is carried by OFDM symbol S2 ₁₇₈, which isthe second OFDM symbol in the sixtieth group of three OFDM symbols andwhich therefore is shown to have the form SF_(1.60)′. In addition, inthe preferred embodiment each code digit C_(y) is added as an additionalphase component to the control data points of the corresponding OFDMsymbol, as shown in FIG. 6 with an indication of +jC_(y). As a result,by adding this code term to the terms of Equation 2, then for the secondOFDM symbol in the R^(th) group of three OFDM symbols, each control datapoint in that OFDM symbol is the same, and may be represented as in thefollowing Equation 12:e^(j(φ) ^(x) ^(+hφ) ^(x) ^(+C) ^(R) ⁾  Equation 12The effect of the result in Equation 12 is further understood withreference to the preferred synchronization to a received superframe, asis described immediately below.

FIG. 7 illustrates a functional block diagram of a synchronizationcircuit 50 of the preferred embodiment and as implemented as part ofsynchronization block 34 in FIG. 2. Looking to FIG. 7, synchronizationcircuit 50 includes a first sample set SMP₁ representing the controldata points for a number of OFDM symbols equal to the length of thepreferred superframe, other than the first control data point of eachsuch OFDM symbol. For consistency with the previous example, therefore,the length of first sample SMP₁ equals 180 to match the length ofsuperframe SPRF of FIG. 4. Synchronization circuit 50 further includes asecond sample set SMP₂, where SMP₂ is the same length as SMP₁ andrepresents the complex conjugates of the respective control data pointsin the OFDM symbols that immediately-precede the OFDM symbols in firstsample set SMP₁. In other words, the complex conjugates in sample setSMP₂ are, in effect, the control data points from a one OFDM symbol timeshift backward with respect to the respective control data points of theset of 180 OFDM symbols in sample set SMP₁. Synchronization circuit 50further includes a product store 52, which by the illustration of FIG. 7is intended to depict the results of a multiplication of each value insample set SMP₁ with the respective value in sample set SMP₂. Forexample, product store 52 stores a result, R₀, as the product of S2 ₁with S2 ₀*, product store 52 stores a result, R₁, as the product of S22with S2 ₁*, and so forth up through result R₁₇₉, which is the product ofS2 ₀ (from the next superframe) with S2 ₁₇₉*. The output of every thirdresult is coupled to a correlator 54, and more particularly as a firstmultiplicand to a respective multiplier within correlator 54, where eachsuch multiplier receives as a second multiplicand an exponential with anegative value of a different one of the 60 code digits from Equation11. For example, result R₂ is connected as a multiplicand to amultiplier M₁ that receives the value e^(−jC) ¹ as a secondmultiplicand, result R₅ is connected as a multiplicand to a multiplierM₂ that receives the value e^(−jC) ² as a second multiplicand, and soforth through result R₁₇₉, which is connected as a multiplicand to amultiplier M₆₀ that receives the value e^(−jC) ⁶⁰ as a secondmultiplicand. Finally, the outputs of all multipliers M₁ through M₆₀ areconnected to a summer 56, which provides an output signal SUM_(OUT).

The operation of synchronization circuit 50 according to the preferredembodiment is now described. First, from the connections describedabove, one skilled in the art will appreciate that the multiplication ofsample set SMP₁ with sample set SMP₂ provides results R₀ through R₁₇₉relating to the control data points in a first stream of 180 OFDMsymbols as multiplied times the complex conjugates of the respectivecontrol data points in a second stream of 180 OFDM symbols, where thefirst and second streams are shifted in time relative to one another byone OFDM symbol. Thus, in this sense alone, this multiplication operateswith respect to a group of 180 OFDM symbols in the same manner asdescribed in the prior art to identify the location of CPS information;however, in the preferred embodiment, the result instead bears on thesynchronization of superframe SPRF as a whole, as is now explored ingreater detail. Once results R₀ through R₁₇₉ are obtained, correlator 54determines a correlation of the 60 digit code from Equation 11 withrespect to those results, where this determination is achieved bymultiplying every third location in the result times a corresponding oneof the digits from the 60 digit code and summing the result to outputSUM_(OUT). For sake of reference, let this first correlation sum, takenwith respect to the 60 results R₂, R₅, . . . R₁₇₉, be represented asSUM_(OUT1). Next, as shown by dashed arrows with respect to the inputsof each multiplier M₁ through M₆₀ and the 60 results R₁, R₄, . . . R₁₇₈,correlator 54 shifts to the left by one result location and theoperation is repeated so that a second correlation sum is taken withrespect to those 60 results R₁, R₄, . . . R₁₇₈, and let this sum berepresented as SUM_(OUT2). Finally, as also shown by dashed arrows withrespect to the inputs of each multiplier M₁ through M₆₀ and the 60results R₀, R₃, . . . R₁₇₇, correlator 54 shifts to the left by oneresult location and the operation is repeated so that a thirdcorrelation sum is taken with respect to the 60 results R₀, R₃, . . .R₁₇₇, and let this sum be represented as SUM_(OUT3). The sum valuesSUM_(OUT1), SUM_(OUT2), and SUM_(OUT3) are then stored, for use asfurther described below

After sum values SUM_(OUT1), SUM_(OUT2), and SUM_(OUT3) are determinedand stored, sample sets SMP₁ and SMP₂ are also shifted by three OFDMsymbols and the above process is repeated in comparable fashion todetermine sum values SUM_(OUT4), SUM_(OUT5), and SUM_(OUT6) in the samemanner as above, that is, by correlator 54 first operating in theposition shown in FIG. 7 to obtain SUM_(OUT4), and then shifting once tothe left for each respective determination of SUM_(OUT5) and SUM_(OUT6).The above-described process continues, wherein one skilled in the artwill therefore appreciate that for each shift of three symbols forsample sets SMP₁ and SMP₂, there is then three sum determinations bycorrelator 54 at the three different above-described shifting positionsi with respect to results R_(i), R_(i+3), . . . R_(177+i). Given theabove, one skilled in the art will appreciate that once the sample setsSMP₁ and SMP₂ have shifted a total of 60 times (i.e., at three symbolsfor each shift and thereby corresponding to the 180 OFDM symbols in asuperframe), correlator 54 has determined a total of 180 (i.e.,3*60=180) values for SUM_(OUT), and in doing so it has effectivelyperformed a sliding window correlation of the Equation 11 code with thecontrol data points of the incoming OFDM symbols. Accordingly, since all180 different possibilities have been evaluated for the 180 OFDM symbolsin the superframe, and assuming a good autocorrelation property of theEquation 11 code, then for reasons detailed below one noticeable maximumshould occur across the 180 values for SUM_(OUT). This maximum,therefore, indicates an alignment of the values in sample set SMP₁ withthe Equation 11 code, that is, when the values as stored in sample setSMP₁ give rise to the maximum peak, it is then known that sample setSMP₁ at the time corresponding to the location when it provided themaximum stored the superframe in its entirety from beginning to end (asopposed to one portion of one superframe and another portion of aprevious or subsequent superframe). In other words, at that point,proper synchronization of the entire superframe is detected. Naturally,therefore, this detection provides the boundaries of the beginning andend of the superframe.

To further appreciate the result of synchronization circuit 50, notethat it achieves, in the preferred embodiment, a complex conjugatemultiplication between immediately-successive OFDM symbols as describedearlier with respect to the prior art location of CPS, but here thismultiplication relates instead to the inventive OFDM and superframeformat. Specifically, recall that the prior art multiplication sought tolocate two successive OFDM symbols of the form SF₁. However, asexplained above with reference to Equation 12, unlike the prior art, thepreferred embodiment does not use the same data on the control datapoints for each two of the first three OFDM symbols in a sequence, butrather, it adds a different code digit to the control points of eachsecond OFDM symbol in a group of three OFDM symbols. Looking in greaterdetail at this distinction, Equation 3 depicts the four control datapoints as appear in each first OFDM symbol in a group of three OFDMsymbols according to the preferred embodiment, which is the same asshown in the form SF₁ of the prior art. However, in contrast, Equation12 may be expanded to demonstrate the three latter control data points(i.e., ignoring the first control data point since it does not carrycontrol information) in the second OFDM symbol in a group of threeaccording to the preferred embodiment (e.g., form SF_(1.1)′), which havean added synchronization code digit C₁, as is shown in the followingEquation 13:e^(j(φ) ² ^(+hφ) ² ^(+C) ¹ ⁾, e^(j(φ) ³ ^(+hφ) ³ ^(+C) ¹ ⁾, e^(j(φ) ⁴^(+hφ) ⁴ ^(+C) ¹ ⁾  Equation 13Again ignoring the first control data point in the OFDM symbols, thenfrom Equations 13 and 3, it may be appreciated that when synchronizationcircuit 50 operates to multiply each data point in one of theseEquations times the complex conjugate of the corresponding data point inthe other Equation, then the result is as shown in the followingEquation 14:e^(j(C) ¹ ⁾, e^(j(C) ¹ ⁾, e^(j(C) ¹ ⁾  Equation 14In other words, Equation 14 demonstrates that by including the codedigit in the control data points of each second OFDM symbol in a groupof three OFDM symbols as shown in Equation 12, then the complexconjugate operation when that second OFDM symbol is multiplied times thefirst OFDM symbol in the group provides just the value of the code digitC₁. Naturally, this same principle applies to each first and second OFDMsymbol in a group of three OFDM symbols, that is, as between a form SF₁′and a form SF_(1.R)′ OFDM symbol. Moreover, the complex conjugateoperation across all 60 groups of three OFDM symbols therefore providesthe entire synchronization code of Equation 11. Further, when thecomplex conjugate operation is performed with respect to other OFDMsymbols, such as between the second and third OFDM symbol in a group(i.e., forms SF_(1.R)′ and SF_(2.x), respectively) or between the thirdOFDM symbol in one group and the immediately-following first OFDM symbolin the next group (i.e., forms SF_(2.x) and SF₁′ respectively), then theresult will include a considerable phase component and therefore isreadily distinguishable from the result that occurs as shown by way ofexample in the preceding Equation 14.

Given the above, one skilled in the art will appreciate that in the 180values in product store 52, every third value therein will store a digitof the 60 digit code, but there are two unknowns. First, it is unknownwhether these digits will be in the proper order from the first to lastof the 60 digits, and second it is unknown what is the offset, if any,from result Ro in which case the 60 digits are stored. With respect tothe latter, if the offset is one, then one of each of the 60 differentcodes, in unknown order, will appear in results R₁, R₄, . . . R₁₇₈, andfor an offset of two, then one of each of the 60 different codes, inunknown order, will appear in results R₂, R₅, . . . R₁₇₉, or an offsetof zero, then one of each of the 60 different codes, in unknown order,will appear in R₀, R₂, . . . R₁₇₇. The further operation of correlator54 can determine both the offset and find the instance of whensuperframe SPRF is properly aligned in sample set SMP₂. Particularly,when the 60 digits of the code are in sequential order within productstore 52, then digit C₁ will be in one of results R₀ through R₂, digitC₂ will be in one of results R₃ through R₅, and so forth. Accordingly,when correlator 54 determines its first value of SUM_(OUT) at an offsetof two (i.e., for results R₁, R₄, . . . R₁₇₈), if the 60 digits arealigned in those locations then note that the first code e^(jC) ¹ , asstored in result R₂, will be multiplied times its complex conjugate ofe^(−jC) ¹ ; as a result, the phase terms will cancel leaving only anamplitude result. In contrast, if there is no alignment, then the resultof the multiplication will include a non-negligible phase amount,thereby producing a lesser value for the output of multiplier M₁. Thissame effect occurs across all of multipliers M₁ through M₆₀ and, hence,is also reflected in the sum of the outputs of those multipliers, thatis, in SUM_(OUT). Accordingly, once the 60 digits of the code arelocated in order and in product store 52, the value of SUM_(OUT) is amaximum. Recalling from above that synchronization circuit 50 stores all180 different possible values of SUM_(OUT), then the one providing amaximum will correspond to the time when a superframe was properlyaligned (i.e., synchronized) within sample set SMP₂.

Further in connection with synchronization circuit 50, recall the abovefunction assumes that the 60 digit code of Equation 11 provides anacceptable autocorrelation property. Toward this end, in the preferredembodiment the 60 digit code is derived from a length 64 constantamplitude zero auto-correlation (“CAZAC”) sequence, where CAZAC codes intheir full-length entirety are known in the correlation art. However,given the preferred embodiment need for a 60 digit code as opposed tothe 64 digits provided in the known CAZAC codes, it was endeavored inconnection with the preferred embodiments to evaluate what modificationto a 64 digit code provided an optimal superframe synchronization.Toward this end, it was determined that the optimal correlation wasachieved by eliminating the last 4 digits from a known 64 digit CAZACcode, as opposed to some other alteration as many were considered suchas removing single or multiple digits at different locations within the64 different digits of the code. Accordingly, in the preferredembodiment, a respective one of each of the 60 digits is modulated onthe control data points in each respective form SF_(1.x)′ symbol of FIG.6 and in the sequence shown in the following Table 2, where each valueis an integer multiplied times a value

$\frac{\pi}{4}.$

TABLE 2 Sequence Digit 1 0 2 1 3 2 4 3 5 4 6 5 7 6 8 7 9 0 10 2 11 4 126 13 0 14 1 15 3 16 6 17 0 18 3 19 6 20 0 21 3 22 7 23 1 24 5 25 0 26 427 0 28 3 29 0 30 3 31 0 32 3 33 0 34 5 35 1 36 7 37 3 38 1 39 6 40 2 410 42 6 43 3 44 1 45 0 46 6 47 3 48 2 49 0 50 7 51 6 52 5 53 3 54 2 55 256 0 57 0 58 0 59 0 60 0

For example, for the first digit in Table 2 of 0, the control datapoints in form SF_(1.1)′ include a phase shift of

${{0*\frac{\pi}{4}} = 0},$while for the second digit in Table 2 of 0, the control data points inform SF_(1.2)′ include a phase shift of

${{1*\frac{\pi}{4}} = \frac{\pi}{4}},$and for the sixtieth digit in Table 2 which is also 0, the control datapoints in form SF_(1.60)′ also include a phase shift

${0*\frac{\pi}{4}} = 0.$One skilled in the art may readily ascertain the remaining phase shiftsfrom Table 2

From the above, it may be appreciated that the above embodiments providean OFDM wireless communication system wherein differing sets of datapoints are defined within a single OFDM symbol, where each set may bemodulated by a different group of modulation parameters. As a result,different receivers (e.g., WMs) can receive signals in the samebandwidth and from the same WATS, but due to the differing controlparameters, greater overall signal performance can be achieved as to allreceivers by customizing the parameters per receiver or sets ofreceivers. For example, different receivers can receive differentconstellation densities and/or FEC parameters in the downstreamdirection, as specified to the receivers in the AMOD information. Tofurther implement these benefits, for example, at start-up, each WM maydetermine which constellation and FEC parameters it may accommodate andestablish this information in its user profile. Thereafter, the WATS maydynamically read this information and adapt the AMOD information intogroups to correspond to the different parameters specified in the userprofiles of the different WMs. In response, the WATS adapts themodulation and/or FEC parameters to optimize cells dynamically by usingdifferent downstream AMOD for different groups of WMs, therebyincreasing cell capacity. Further, the preferred embodiment has beenshown to have different variations, where one skilled in the art willappreciate still others. Consequently, while the present embodimentshave been described in detail, various substitutions, modifications oralterations could be made to the descriptions set forth above withoutdeparting from the inventive scope which is defined by the followingclaims.

1. A wireless transmitter, comprising: circuitry for providing aplurality of control bits; circuitry for providing a plurality of userbits; circuitry for modulating the plurality of control bits and theplurality of user bits into a stream of complex symbols; circuitry forconverting the stream of complex symbols into a parallel plurality ofcomplex symbol streams; circuitry for performing an inverse fast Fouriertransform on the parallel plurality of complex symbol streams to form aparallel plurality of OFDM symbols; and circuitry for converting theparallel plurality of OFDM symbols into a serial stream of OFDM symbols,wherein the serial stream consists of an integer N+1 OFDM symbols;wherein each OFDM symbol in the serial stream of OFDM symbols comprisesa plurality of data points; and wherein selected OFDM symbols in theserial stream of OFDM symbols each comprise control data points carryinga portion of a synchronization code wherein N+1 is a multiple of three;and wherein the selected OFDM symbols are each a second OFDM symbol in asequence of three OFDM symbols in the serial stream.
 2. The wirelesstransmitter of claim 1 wherein each portion of the synchronization codeis represented as an exponential phase shift on the control data pointscarrying a portion of a synchronization code.
 3. The wirelesstransmitter of claim 1 wherein, for each sequence of three OFDM symbolsin the serial stream, a first OFDM symbol in the sequence and a thirdOFDM symbol in the sequence do not include any portion of thesynchronization code.
 4. The wireless transmitter of claim 3: whereinN+1=180; and wherein the synchronization code consists of 60 digits. 5.The wireless transmitter of claim 4 wherein the synchronization codecomprises a portion of a 64-bit constant amplitude zero auto-correlationsequence.
 6. The wireless transmitter of claim 4 wherein thesynchronization code consists of a last 60 bits of a 64-bit constantamplitude zero auto-correlation sequence.
 7. The wireless transmitter ofclaim 6 wherein each portion of the synchronization code is representedas a different one of the 60 bits times a value $\frac{\pi}{4}$ andrepresented as an exponential phase shift on the control data pointscarrying a portion of a synchronization code.
 8. The wirelesstransmitter of claim 6 wherein the last 60 bits Consist of a stream 0 12 3 4 5 6 7 0 2 4 6 0 1 3 6 0 3 6 0 3 7 1 5 0 4 0 3 0 3 0 3 0 5 1 7 3 16 2 0 6 3 1 0 6 3 2 0 7 6 5 3 2 2 0 0 0 0
 0. 9. The wireless transmitterof claim 1 wherein the synchronization code comprises a portion of a64-bit constant amplitude zero auto-correlation sequence.
 10. Thewireless transmitter of claim 1: wherein the selected OFDM symbols areeach a second OFDM symbol in a sequence of three OFDM symbols in theserial stream; and wherein, for each sequence of three OFDM symbols inthe serial stream, a first OFDM symbol in the sequence and a third OFDMsymbol in the sequence do not include any portion of the synchronizationcode.
 11. The wireless transmitter of claim 1: wherein N+1=180; andwherein the synchronization code consists of 60 digits.
 12. The wirelesstransmitter of claim 11 wherein the synchronization code comprises aportion of a 64-bit constant amplitude zero auto-correlation sequence.13. The wireless transmitter of claim 12 wherein the synchronizationcode consists of a last 60 bits of a 64-bit constant amplitude zeroauto-correlation sequence.
 14. The wireless transmitter of claim 13wherein each portion of the synchronization code is represented as adifferent one of the 60 bits times a value $\frac{\pi}{4}$ andrepresented as an exponential phase shift on the control data pointscarrying a portion of a synchronization code.
 15. The wirelesstransmitter of claim 1 wherein the synchronization code consists of alast 60 bits of a 64-bit constant amplitude zero auto-correlationsequence.
 16. The wireless transmitter of claim 15 wherein each portionof the synchronization code is represented as a different one of the 60bits times a value $\frac{\pi}{4}$ and represented as an exponentialphase shift on the control data points carrying a portion of asynchronization code.
 17. The wireless transmitter of claim 1 whereinthe synchronization code consists of a stream 0 1 2 3 4 5 6 7 0 2 4 6 01 3 6 0 3 6 0 3 7 1 5 0 4 0 3 0 3 0 3 0 5 1 7 3 1 6 2 0 6 3 1 0 6 3 2 07 6 5 3 2 2 0 0 0 0
 0. 18. A wireless receiver, comprising: at least oneantenna for receiving a wireless signal, the signal comprising a serialstream of OFDM symbols, wherein each OFDM symbol in the serial stream ofOFDM symbols comprises a plurality of data points; and circuitry fordetecting a synchronization code from data points in selected OFDMsymbols in the serial stream of OFDM symbols, wherein the selected OFDMsymbols in the serial stream of OFDM symbols each comprise data pointscarrying a portion of a synchronization code wherein each portion of thesynchronization code is represented as an exponential phase shift on thedata points carrying a portion of a synchronization code, wherein theserial stream consists of an integer N+1 OFDM symbols and wherein N+1 isa multiple of three, and wherein the selected OFDM symbols are each asecond OFDM symbol in a sequence of three OFDM symbols in the serialstream.
 19. The wireless receiver of claim 18 wherein the circuitry fordetecting comprises: circuitry for multiplying selected data points ineach of a first group of OFDM symbols from the serial stream times acomplex conjugate of respective selected data points in a second groupof OFDM symbols from the serial stream, wherein the first group andsecond group are time shifted by one OFDM symbol with respect to oneanother; and circuitry for determining a correlation between a storedcode and selected results from the circuitry for multiplying.
 20. Thewireless receiver of claim 19: wherein N+1=180; and wherein thesynchronization code consists of 60 digits.
 21. The wireless receiver ofclaim 20 wherein the synchronization code comprises a portion of a64-bit constant amplitude zero auto-correlation sequence.
 22. Thewireless receiver of claim 21 wherein the synchronization code consistsof a last 60 bits of a 64-bit constant amplitude zero auto-correlationsequence.
 23. The wireless receiver of claim 22 wherein each portion ofthe synchronization code is represented as a different one of the 60bits times a value $\frac{\pi}{4}$ and represented as an exponentialphase shift on the data points carrying a portion of a synchronizationcode.
 24. The wireless receiver of claim 23 wherein the last 60 bitsconsist of a stream 0 1 2 3 4 5 6 7 0 2 4 6 0 1 3 6 0 3 6 0 3 7 1 5 0 40 3 0 3 0 3 0 5 1 7 3 1 6 2 0 6 3 1 0 6 3 2 0 7 6 5 3 2 2 0 0 0 0
 0. 25.The wireless receiver of claim 18 wherein, for each sequence of threeOFDM symbols in the serial stream, a first OFDM symbol in the sequenceand a third OFDM symbol in the sequence do not include any portion ofthe synchronization code.
 26. The wireless receiver of claim 18 whereinthe circuitry for detecting comprises: circuitry for multiplyingselected data points in each of a first group of OFDM symbols from theserial stream times a complex conjugate of respective selected datapoints in a second group of OFDM symbols from the serial stream, whereinthe first group and second group are time shifted by one OFDM symbolwith respect to one another; and circuitry for determining a correlationbetween a stored code and selected results from the circuitry formultiplying.
 27. The wireless receiver of claim 18: wherein the serialstream consists of an integer N+1 OFDM symbols and wherein N+1 is amultiple of three; wherein N+1=180; wherein the synchronization codeconsists of 60 digits; and wherein the selected OFDM symbols are each asecond OFDM symbol in a sequence of three OFDM symbols in the serialstream.
 28. The wireless receiver of claim 27 wherein the circuitry fordetecting comprises: circuitry for multiplying selected data points ineach of a first group of OFDM symbols from the serial stream times acomplex conjugate of respective selected data points in a second groupof OFDM symbols from the serial stream, wherein the first group andsecond group are time shifted by one OFDM symbol with respect to oneanother; and circuitry for determining a correlation between a storedcode and selected results from the circuitry for multiplying.
 29. Thewireless receiver of claim 18 wherein the synchronization code comprisesa portion of a 64-bit constant amplitude zero auto-correlation sequence.30. The wireless receiver of claim 18 and further comprising: circuitryfor converting the serial stream of OFDM symbols into a plurality ofparallel streams of OFDM symbols; and circuitry for performing a fastFourier transform on the plurality of parallel streams of OFDM symbolsto form a parallel plurality of complex symbols.
 31. The wirelessreceiver of claim 30 and further comprising: circuitry for convertingthe parallel plurality of complex symbols into a serial stream ofcomplex symbols; and circuitry for demodulating the serial stream ofcomplex symbols.
 32. A communication device, comprising: a fast Fouriertransform configured to perform an inverse fast Fourier transform on aparallel plurality of complex symbol streams to form a parallelplurality of OFDM symbols; and circuitry for converting the parallelplurality of OFDM symbols into a serial stream of OFDM symbols, whereinthe serial stream consists of an integer N+1 OFDM symbols, N+1 is amultiple of three, and wherein each a second OFDM symbol in a sequenceof three OFDM symbols in the serial stream of OFDM symbols comprisecontrol data paints carrying a portion of a synchronization code.
 33. Acommunication device according to claim 32, further comprising:circuitry for providing a plurality of control bits; circuitry forproviding a plurality of user bits; circuitry for modulating theplurality of control bits and the plurality of user bits into a streamof complex symbols; and circuitry for converting the stream of complexsymbols into the parallel plurality of complex symbol streams, whereineach OFDM symbol in the serial stream of OFDM symbols comprises aplurality of data points.
 34. A communication device according to claim32 wherein each portion of the synchronization code is represented as anexponential phase shift on the control data points carrying a portion ofa synchronization code.
 35. A communication device according to claim32, wherein, for each sequence of three OFDM symbols in the serialstream, a first OFDM symbol in the sequence and a third OFDM symbol inthe sequence do not include any portion of the synchronization code. 36.A communication device according to claim 32, wherein thesynchronization code comprises a portion of a 64-bit constant amplitudezero auto-correlation sequence.
 37. A communication device, comprising:circuitry for receiving a serial stream of OFDM symbols, wherein theserial stream consists of an integer N+1 OFDM symbols and wherein N+1 isa multiple of three; and circuitry for detecting a portion ofsynchronization code from data points in each a second OFDM symbol in asequence of three OFDM symbols in the serial stream.
 38. A communicationdevice according to claim 37, wherein each portion of thesynchronization code is represented as an exponential phase shift on thedata points carrying a portion of a synchronization code.
 39. Acommunication device according to claim 37, wherein the circuitry fordetecting comprises: circuitry for multiplying selected data points ineach of a first group of OFDM symbols from the serial stream times acomplex conjugate of respective selected data points in a second groupof OFDM symbols from the serial stream, wherein the first group andsecond group are time shifted by one OFDM symbol with respect to oneanother; and circuitry for determining a correlation between a storedcode and selected results from the circuitry for multiplying.
 40. Acommunication device according to claim 37, wherein the communicationdevice is a wireless receiver; and the circuitry for receiving theserial stream of OFDM symbols comprises at least one antenna.
 41. Acommunication device according to claim 37, wherein for each sequence ofthree OFDM symbols in the serial stream, a first OFDM symbol in thesequence and a third OFDM symbol in the sequence do not include anyportion of the synchronization code.
 42. A communication deviceaccording to claim 37, further comprising: circuitry for converting theserial stream of OFDM symbols into a plurality of parallel streams ofOFDM symbols; circuitry for performing a fast Fourier transform on theplurality of parallel streams of OFDM symbols to form a parallelplurality of complex symbols; circuitry for converting the parallelplurality of complex symbols into a serial stream of complex symbols;and circuitry for demodulating the serial stream of complex symbols.